Distribution and significance of hydrothermal alteration minerals in the Tuzla hydrothermal system, Çanakkale, Turkey

Hydrothermal alteration zones have been investigated by X-ray diffraction, mineralogical–petrographical techniques, and geochemical analysis. Examination of cores and cuttings from two drill sites, obtained from a depth of about 814–1020 m, show that the hydrothermal minerals occuring in the rock include: K-feldspar, albite, chlorite, alunite, kaolinite, smectite, illite, and opaque minerals.In the studied area, silicified, smectite, illite, alunite, and opal zones have been recognized. These alteration mineral assemblages indicate that there are geothermal fluids, which have temperatures of 150–220°C in the reservoir.The distribution of the hydrothermal minerals shows changes in the chemical composition of the hydrothermal fluid, which are probably due not only to interaction with host rock, but also to dilution of the Na–K–Cl-rich hydrothermal fluid of the deep reservoir by cold sea water at shallow levels. Geochemical analyses of the solid and liquid phases indicate that the hydrothermal fluids of the Tuzla geothermal system are in equilibrium with alteration products.The tectonic structure of the studied area is caused by NW–SE and NE–SW directional forces. The volcanic rocks where hydrothermal zones are observed in the studied area are of Lower–Middle Miocene age comprise latite, andesite, dacite, rhyolite-type lavas, tuff, and ignimbrites.     

Geochemistry of the magmatic–hydrothermal system of Kawah Ijen volcano, East Java, Indonesia

Samples from Kawah Ijen crater lake, spring and fumarole discharges were collected between 1990 and 1996 for chemical and isotopic analysis. An extremely low pH (<0.3) lake contains SO₄–Cl waters produced during absorption of magmatic volatiles into shallow ground water. The acidic waters dissolve the rock isochemically to produce “immature” solutions. The strong D and ¹⁸O enrichment of the lake is mainly due to enhanced evaporation at elevated temperature, but involvement of a magmatic component with heavy isotopic ratios also modifies the lake D and ¹⁸O content. The large Δ_SO4–S⁰ (23.8–26.4‰) measured in the lake suggest that dissolved SO₄ forms during disproportionation of magmatic SO₂ in the hydrothermal conduit at temperatures of 250280°C. The lake δ¹⁸O_SO4 and δ¹⁸O_H2O values may reflect equilibration during subsurface circulation of the water at temperatures near 150°C. Significant variations in the lake's bulk composition from 1990 to 1996 were not detected. However, we interpret a change in the distribution and concentration of polythionate species in 1996 as a result of increased SO₂-rich gas input to the lake system.Thermal springs at Kawah Ijen consist of acidic SO₄–Cl waters on the lakeshore and neutral pH HCO₃–SO₄–Cl–Na waters in Blawan village, 17 km from the crater. The cation contents of these discharges are diluted compared to the crater lake but still do not represent equilibrium with the rock. The SO₄/Cl ratios and water and sulfur isotopic compositions support the idea that these springs are mixtures of summit acidic SO₄–Cl water and ground water.The lakeshore fumarole discharges (T=170245°C) have both a magmatic and a hydrothermal component and are supersaturated with respect to elemental sulfur. The apparent equilibrium temperature of the gas is 260°C. The proportions of the oxidized, SO₂-dominated magmatic vapor and of the reduced, H₂S-dominated hydrothermal vapor in the fumaroles varied between 1979 and 1996. This may be the result of interaction of SO₂-bearing magmatic vapors with the summit acidic hydrothermal reservoir. This idea is supported by the lower H₂S/SO₂ ratio deduced for the gas producing the SO₄–Cl reservoir feeding the lake compared with that observed in the subaerial gas discharges. The condensing gas may have equilibrated in a liquid–vapor zone at about 350°C.Elemental sulfur occurs in the crater lake environment as banded sediments exposed on the lakeshore and as a subaqueous molten body on the crater floor. The sediments were precipitated in the past during inorganic oxidation of H₂S in the lake water. This process was not continuous, but was interrupted by periods of massive silica (poorly crystallized) precipitation, similar to the present-day lake conditions. We suggest that the factor controlling the type of deposition is related to whether H₂S- or silica-rich volcanic discharges enter the lake. This could depend on the efficiency with which the lake water circulates in the hydrothermal cell beneath the crater. Quenched liquid sulfur products show δ³⁴S values similar to those found in the banded deposits, suggesting that the subaqueous molten body simply consists of melted sediments previously accumulated at the lake bottom.     

Fluids in early stage hydrothermal alteration of high-sulfidation epithermal systems: A view from the Vulcano active hydrothermal system (Aeolian Island,Italy)

High-sulfidation (HS) epithermal systems have elements in common with passively degassing volcanoes associated with high T, acid fumarole fields or acid crater lakes. They are considered to form in two stages, the first of which involves advanced argillic alteration resulting from intense, strongly acidic fluid–rock interaction. The La Fossa hydrothermal system (Vulcano Island) represents a classic example of such an active HS system and can be considered as a modern analogue of this early stage of alteration, resulting in a core of intense silicic (90–95% pure SiO₂) alteration surrounded by alunitic alteration zones.     

The hydrothermal vent system of Mount Ruapehu,New Zealand — a high frequency MT survey of the summit plateau

High frequency magnetotelluric (MT) measurements made on the summit plateau of Mount Ruapehu, some 1 km to the north of the presently active vent beneath Crater Lake, have been used to derive the electrical resistivity structure associated with the volcanic hydrothermal vent system. The entire summit plateau area is underlain at shallow depth by low resistivity which is inferred to be the result of hydrothermal alteration caused by rising volcanic gases mixing with local groundwater. Two areas of localised higher resistivity, one between 200 and 500 m depth beneath the central part of the plateau, and one at a depth of 1000 m below the northern part of the plateau, are interpreted as being the result of hydrothermal alteration at higher temperature forming chlorite dominated alteration products. These regions are believed to represent the locations of further heat pipes within the volcanic system. Both correlate with the locations of eruption centres on Ruapehu active within the last 10 ka.     

Water/rock interaction in the Kızılcahamam Geothermal Field, Galatian Volcanic Province (Turkey): a modelling study of a geothermal system for reinjection well locations

The Kızılcahamam geothermal field is emplaced in Tertiary-aged volcanic units 70 km NW of Ankara (Turkey). Data for this low-temperature (74–86°C) geothermal field regarding the fracture zone system were obtained from surface manifestations (hot springs, alteration zones), five exploration wells (MTA-2, -3, -4, -5, -6) and two production wells (KHD-1, MTA-1). The Kızılcahamam reservoir developed along the Kızılcahamam fault zone and so the production wells (180–1556 m) effect each other due to their limited separation. Meteoric water enters from a recharge area NE of Kızılcahamam, circulates and gains heat through the fault zone, and discharges to the surface.Detailed petrographical studies have been carried out with samples taken from surface rocks, cores, and cuttings from three wells (KHD-1, MTA-2, -3). X-ray diffraction techniques were also used in the present study. Kaolinite and montmorillonite zones were identified at outcrop samples. Chloritization, clay mineralization, sericitization and carbonization were determined in the ground mass of samples from wells. The observed alteration mineral assemblages indicate that Kızılcahamam geothermal system has been cooling since the alteration minerals formed.The exploration well MTA-3 seems to be more suitable for a reinjection well than the other wells (MTA-2, -6), even if the cost of surface piping to transport the waste water to MTA-3 is higher than to another well (MTA-6).     

Presence of native gold and tellurium in the active high-sulfidation hydrothermal system of the La Fossa volcano (Vulcano, Italy)

Grains of native gold and tellurium were found in siliceous hydrothermally altered rocks in the high-temperature (170–540°C) fumarolic field of the La Fossa volcano (Island of Vulcano). In addition to Au and Te, Pb–Bi sulfides (cannizzarite) and Tl-bromide chloride were found as sublimates in the hottest fumarolic vents of the crater rim. The chemical composition of altered rocks associated with sublimate deposition indicate the presence of a significant concentration of Te (up to 75 ppm), while gold concentrations are very low (<9 ppb). Pb, Bi and Tl are strongly enriched in the hottest and less oxidized fumarolic vents, reaching concentrations of 2186, 146 and 282 ppm, respectively. These elements are transported (generally as chloride complexes) to the surface by volcanic gases, and several of these (Bi, Te, Tl) are originated from magma degassing. The silicic alteration is produced by the flow of fluids with pH<2. High acidity results from introduction of magmatic gases such as SO₂, HCl and HF released by the shallow magmatic reservoir of La Fossa volcano. The silicic alteration found at Vulcano may represent an early stage of the `vuggy silica' facies which characterizes the high-sulfidation epithermal ore deposits, confirming the analogies existing between this type of ore deposit and magmatic-hydrothermal systems associated with island-arc volcanoes.     

Near‐infrared spectroscopy of a hydroecological indicator: new tool for determining sustainable yield for Floridan aquifer system

Pond‐cypress (Taxodium ascendens Brong.) is a dominant canopy species in depressional wetlands of the south‐eastern Coastal Plain. Unsustainable withdrawals from the karst Floridan aquifer system have caused premature decline and death of pond‐cypress trees, presumably owing to altered hydroperiods (which alter the flow of water and nutrients in trees). There has been no scientifically based means to determine sustainable yield from this regional aquifer system or to detect early stages of physical/ecological damage associated with groundwater mining and aquifer storage and recovery (ASR, which also can alter natural hydroperiods). In this study, the relationship between visual symptoms (indicators) of stress or premature decline, and spectral reflectance was evaluated using dried, milled branch tips collected from natural stands of mature pond‐cypress. Depressional systems evaluated represented four of the six aquifer system subregions where subsurface perturbations from groundwater mining: (i) were presumed not to be occurring (reference wetlands); (ii) may be occurring but are not documented; and (iii) have been confirmed. Sampled trees were assigned to one of three stress classes (1, no/minimal; 2, moderate; 3, severe) based on the visual indicators. Partial least squares–linear discriminant analysis of second derivative spectral transformations in the visible/shortwave near‐infrared (NIR) region (400–1100 nm) and the NIR region (1100–2500 nm) was used to evaluate the samples in assigned classes. Class 1 samples were discriminated from combined class 2 and 3 samples in the NIR region with 100% and 97% accuracy for consecutive winter sample periods (before bud‐break). The percentage of correctly classified samples in this spectral region was lower (85%) for summer samples (full leaf‐out). Second‐derivative models for the NIR region developed from the winter data sets predicted assigned classes for alternate winter's samples with an accuracy of 97% and 100%. High correlation between spectral reflectance of dried, milled branch tips collected from mature pond‐cypress in winter and visual indicators of premature decline suggests in situ pond‐cypress are hydroecological indicators of anthropogenic subsurface hydroperiod perturbations. This approach provides objective means for early detection of unsustainable aquifer yield and adverse impacts from ASR activities in the south‐eastern Coastal Plain. Used in conjunction with hydrological monitoring and modelling, the hydroecological indicators should provide the means with which sustainable yield in the south‐eastern Coastal Plain can be achieved and maintained. Copyright © 2003 John Wiley & Sons, Ltd.     

Hydrothermal alteration of plagioclase and growth of secondary feldspar in the Hengill Volcanic Centre, SW Iceland

Dissolution of igneous feldspar and the formation and occurrence of secondary feldspar in tholeiitic basalts from the Hengill volcanic centre, in SW Iceland was studied by microprobe analysis of cuttings from two ca. 2000 m deep geothermal wells. Well NG-7 in Nesjavellir represents a geothermal system in a rift zone where the intensity of young, insignificantly altered intrusions increases with depth. Well KhG-1 in Kolviðarhóll represents the margin of a rift zone where the intensity of intrusives is lower and the intensity of alteration higher. This marginal well represents altered basaltic crust in an early retrograde state. The secondary plagioclase in both wells is mainly oligoclase, occurring in association with K-feldspar and chlorite±actinolite. The texture of this assemblage depends on the lithology and intensity of alteration. In Nesjavellir (NG-7) the composition of secondary albite-oligoclase is correlated with the host-rock composition. This connection is not apparent in more intensely altered samples from Kolviðarhóll (KhG-1). The influence of temperature on composition of secondary Na-feldspar is unclear in both wells although Ca is expected to increase with temperature. Any temperature dependence may be suppressed by the influence of rock composition in Nesjavellir and by retrograde conditions at Kolviðarhóll. The absence of clear compositional gradients between igneous plagioclase and secondary feldspar and between Na-feldspar and K-feldspar suggests that secondary feldspars formed by dissolution precipitation reactions.     

Characteristics,extent and origin of hydrothermal alteration at Mount Rainier Volcano,Cascades Arc,USA: Implications for debris-flow hazards and mineral deposits

Hydrothermal alteration at Mount Rainier waxed and waned over the 500,000-year episodic growth of the edifice. Hydrothermal minerals and their stable-isotope compositions in samples collected from outcrop and as clasts from Holocene debris-flow deposits identify three distinct hypogene argillic/advanced argillic hydrothermal environments: magmatic-hydrothermal, steam-heated, and magmatic steam (fumarolic), with minor superimposed supergene alteration. The 3.8 km³ Osceola Mudflow (5600 y BP) and coeval phreatomagmatic F tephra contain the highest temperature and most deeply formed hydrothermal minerals. Relatively deeply formed magmatic-hydrothermal alteration minerals and associations in clasts include quartz (residual silica), quartz–alunite, quartz–topaz, quartz–pyrophyllite, quartz–dickite/kaolinite, and quartz–illite (all with pyrite). Clasts of smectite–pyrite and steam-heated opal–alunite–kaolinite are also common in the Osceola Mudflow. In contrast, the Paradise lahar, formed by collapse of the summit or near-summit of the edifice at about the same time, contains only smectite–pyrite and near-surface steam-heated and fumarolic alteration minerals. Younger debris-flow deposits on the west side of the volcano (Round Pass and distal Electron Mudflows) contain only low-temperature smectite–pyrite assemblages, whereas the proximal Electron Mudflow and a < 100 y BP rock avalanche on Tahoma Glacier also contain magmatic-hydrothermal alteration minerals that are exposed in the avalanche headwall of Sunset Amphitheater, reflecting progressive incision into deeper near-conduit alteration products that formed at higher temperatures.     

Evolution of a vent-hosted hydrothermal system beneath Ruapehu Crater Lake,New Zealand

A two-year chemical monitoring program of Ruapehu Crater Lake shows that it has evolved considerably since the volcano's more active eruptive periods in the early 1970s. The present pH (20°C) of 0.6 is about one half unit more acid than the baseline values in the 1970s, whereas S/Cl ratios have increased markedly owing in part to declining HCl inputs into the lake, but also to absolute increases in SO₄ levels which now stand at the highest values ever recorded. Increases in K/Mg and Na/Mg ratios over the 20-year period are attributed to hydrothermal reaction processes in the vent which are presently causing dissolution of previously formed alteration phases such as natroalunite. These observations, combined with results of a recent heat budget analysis of the lake, have led to the development of hydrothermal convection model for the upper portion of the vent. Possible vent/lake chemical reaction processes between end member reactants have been modelled with the computer code CHILLER. The results are consistent with the view that variations in lake chemistry, which are initiated by the introduction of fresh magmatic material into the vent, reflect the extent of dissolution reaction progress on the magmatic material and/or its alteration products. The results also provide insights into the role of such vent processes in the formation of high sulfidation-type ore deposits.     

Seismic response of a bridge–soil–foundation system under the combined effect of vertical and horizontal ground motions

Different levels of model sophistication have recently emerged to support seismic risk assessment of bridges, but mostly at the expense of neglecting the influence of vertical ground motions (VGMs). In this paper, the influence of VGMs on bridge seismic response is presented and the results are compared with the case of horizontal‐only excitations. An advanced finite element model that accounts for VGMs is first developed. Then, to investigate the effect of soil–structure interaction (SSI) including liquefaction potential, the same bridge with soil‐foundation and fixed boundary conditions is also analyzed. Results show that the inclusion of the VGMs has a significant influence on the seismic response, especially for the axial force in columns, normal force of bearings, and the vertical deck bending moments. However, VGMs do not have as much influence on the seismic demand of the pile cap displacements or pile maximum axial forces. Also, the significant fluctuation of the column axial force can reduce its shear and flexural capacity, and a heightened reversal of flexural effects may induce damage in the deck. In addition, relative to the fixed base case, SSI effects tend to reduce response quantities for certain ground motions while increasing demands for others. This phenomenon is explained as a function of the frequency content of the ground motions, the shift in natural vertical periods, and the VGM spectral accelerations at higher modes. Moreover, the mechanisms of liquefaction are isolated relative to SSI effects in nonliquefiable soils, revealing the influence of liquefaction on bridge response under VGMs. Copyright © 2012 John Wiley & Sons, Ltd.     

Geochemistry of fluids associated with the 1995–1996 eruption of Mt. Ruapehu, New Zealand: signatures and processes in the magmatic-hydrothermal system

The 1995–1996 eruption of Mt. Ruapehu has provided a number of insights into the geochemical processes operating within the magmatic-hydrothermal system of this volcano. Both pre-eruption degassing of the rising magma and its eventual intrusion into the convective zone of the hydrothermal system beneath the lake were clearly reflected in lake water compositions. The eruptions of September–October 1995 expelled the lake, and provided the first-ever opportunity to characterise gas discharges from this volcano. The fumarolic discharges revealed compositions typical of andesite volcanoes and strong interaction with the enclosing meteoric and hydrothermal system fluids. Some 1.1 MT of SO₂ gas was released from the volcano between September 1995 and December 1996, whereas ca. twice this amount (2.2 MT equivalent SO₂) was erupted as soluble (i.e. leachable) oxyanions of sulphur. Significantly more sulphur was released from the volcano over this period than can be accounted for from the magma volume actually erupted. The evidence suggests that a sizable component of the evolved sulphur was remobilised from the long-lived hydrothermal system within the volcano during the 1995–1996 activity.     

Chemical evolution of thermal waters and changes in the hydrothermal system of Papandayan volcano (West Java,Indonesia) after the November 2002 eruption

Papandayan is a stratovolcano situated in West Java, Indonesia. Since the last magmatic eruption in 1772, only few hydrothermal explosions have occurred. An explosive eruption occurred in November 2002 and ejected ash and altered rocks. The altered rocks show that an advanced argillic alteration took place in the hydrothermal system by interaction between acid fluids and rocks. Four zones of alteration have been defined and are limited in extension and shape along faults or across permeable structures at different levels beneath the active crater of the volcano.     

Regional metamorphism and hydrothermal alteration related to Miocene submarine volcanism ('green tuff') in the Yurihara oil and gas field, northwest Honshu Island, Japan

Abstract Alteration of reservoir rocks in the Yurihara Oil and Gas Field, hereafter referred to as the ‘Yurihara field’, have been examined by using samples from six wells. These rocks are basalts in the lowermost part of the basin-fills (‘green tuff’ Formation). These basalts were produced in many eruptions in a submarine environment during the early to middle Miocene, and they underwent continuous intensive alteration genetically associated with Miocene submarine volcanism. The alteration of the basalts is of two types: low grade metamorphism and hydrothermal. The former belongs to the type of ocean floor metamorphism and comprises two subgroups: zeolite (zone I) and prehnite-pumpellyite (zones IIa: vein and amygdule occurrence, and IIb: replacing plagioclase). The latter is characterized by potassic metasomatism accompanied by adularia, quartz and calcite veins (zones IIIa: center and IIIb: margin of the metasomatism). This overprints the low grade metamorphic alteration. The central zone of hydrothermal alteration coincides with a major estimated fault, so that fluids probably assent along the fault. The basalts erupted during 16.5-15.5 Ma, determined by planktonic foraminifera assemblages of inter-bedded shales, then underwent successive low grade metamorphism. In time, the hydrothermal alteration that overprints low grade metamorphism occurred. Adularia veins of the altered rocks located in the hydrothermal alteration zones (zone IIIa and IIIb) have been dated as 9 Ma determined by the K-Ar method. This fact indicates that the activity of low grade metamorphism had already crossed the peak before hydrothermal alteration occurred at 9 Ma. The shape of isotherms of fluid inclusion homogenization temperatures (Th) and that of isolines of apparent salinity (Tm) almost coincide with each other, and these also coincide with the distribution of hydrothermal alteration (zones IIIa and IIIb). This indicates that the fluid inclusions formed at the same time as ascending fluids produced the potassic metasomatism. The maximum Th of the fluid inclusions is 222°C and Tm indicates trapped fluids of up to 3.3 wt% equivalent NaCl (i.e. almost the same as seawater). A Th versus Tm plot indicates mixing occurred between hydrothermal fluids and formation water that has low salinity. Corrensite and chlorite form veins, and the temperatures of their formation, estimated by the extent of aluminium substitution into the tetrahedral site of chlorite, ranges between 165 and 245°C in the centre of the hydrothermal alteration zone (zone IIIa). This is consistent with the result of Th analyses. The deposition temperature of chlorite associated with prehnite in veins ranges between 190 and 215°C in zones IIa and IIb.     

Field assessment of surface water–groundwater connectivity in a semi‐arid river basin (Murray–Darling,Australia)

In semi‐arid and arid river basins, understanding the connectivity between rivers and alluvial aquifers is one of the key challenges for the management of groundwater resources. The type of connection present (gaining, losing‐connected, transitional and losing‐disconnected) was assessed at 12 sites along six Murray–Darling Basin river reaches. The assessments were made by measuring the hydraulic head in the riparian zone near the rivers to evaluate if the water tables intersected the riverbeds and by measuring fluid pressure (ψ) in the riverbeds. The rationale for the latter was that ψ will always be greater than or equal to zero under connected conditions (either losing or gaining) and always lesser than or equal to zero under losing‐disconnected conditions. A mixture of losing‐disconnected, losing‐connected and gaining conditions was found among the 12 sites. The losing‐disconnected sites all had a riverbed with a lower hydraulic conductivity than the underlying aquifer, usually in the form of a silty clay or clay unit 0.5–2 m in thickness. The riparian water tables were 6 to 25 m below riverbed level at the losing‐disconnected sites but never lower than 1 m below riverbed level at the losing‐connected ones. The contrast in water table depth between connected and disconnected sites was attributed to the conditions at the time of the study, when a severe regional drought had generated a widespread decline in regional water tables. This decline was apparently compensated near losing‐connected rivers by increased infiltration rates, while the decline could not be compensated at the losing‐disconnected rivers because the infiltration rates were already maximal there. Copyright © 2012 John Wiley & Sons, Ltd.     

Input and system identification of the Hualien soil–structure interaction system using earthquake response data

This paper presents an input and system identification technique for a soil–structure interaction system using earthquake response data. Identification is carried out on the Hualien large‐scale seismic test structure, which was built in Taiwan for international joint research. The identified quantities are the input ground acceleration as well as the shear wave velocities of the near‐field soil regions and Young's moduli of the shell sections of the structure. The earthquake response analysis on the soil–structure interaction system is carried out using the finite element method incorporating the infinite element formulation for the unbounded layered soil medium and the substructured wave input technique. The criterion function for the parameter estimation is constructed using the frequency response amplitude ratios of the earthquake responses measured at several points of the structure, so that the information on the input motion may be excluded. The constrained steepest descent method is employed to obtain the revised parameters. The simulated earthquake responses using the identified parameters and input ground motion show excellent agreement with the measured responses. Copyright © 2003 John Wiley & Sons, Ltd.     

Source mechanism of long-period events at Kusatsu–Shirane Volcano, Japan, inferred from waveform inversion of the effective excitation functions

We investigate the source mechanism of long-period (LP) events observed at Kusatsu–Shirane Volcano, Japan, based on waveform inversions of their effective excitation functions. The effective excitation function, which represents the apparent excitation observed at individual receivers, is estimated by applying an autoregressive filter to the LP waveform. Assuming a point source, we apply this method to seven LP events the waveforms of which are characterized by simple decaying and nearly monochromatic oscillations with frequency in the range 1–3 Hz. The results of the waveform inversions show dominant volumetric change components accompanied by single force components, common to all the events analyzed, and suggesting a repeated activation of a sub-horizontal crack located 300 m beneath the summit crater lakes. Based on these results, we propose a model of the source process of LP seismicity, in which a gradual buildup of steam pressure in a hydrothermal crack in response to magmatic heat causes repeated discharges of steam from the crack. The rapid discharge of fluid causes the collapse of the fluid-filled crack and excites acoustic oscillations of the crack, which produce the characteristic waveforms observed in the LP events. The presence of a single force synchronous with the collapse of the crack is interpreted as the release of gravitational energy that occurs as the slug of steam ejected from the crack ascends toward the surface and is replaced by cooler water flowing downward in a fluid-filled conduit linking the crack and the base of the crater lake.     

Depositional age of the Himenoura Group on the Amakusa‐Kamishima area,Kyushu, southwest Japan: Using zircon U–Pb dating of the acidic tuffs

The Upper Cretaceous Himenoura Group in the Amakusa‐Kamishima Island area, southwest Japan is subdivided into the Hinoshima and Amura Formations. In order to determine the numerical depositional age of the formations, zircon U–Pb ages were investigated using laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) for acidic tuff samples from the lower part of the Hinoshima Formation and the upper part of the Amura Formation. Although the two samples contain some accidental zircons, the samples have a definite youngest age cluster and their weighted mean ages are 85.4 ± 1.3 and 81.5 ± 1.1 Ma, respectively (errors are 95 % confidence interval). These age data indicate that the Himenoura Group in the Amakusa‐Kamishima Island area was deposited mainly in the early Santonian to early Campanian which is consistent with biostratigraphic ages. Additionally, zircon age distributions of the two tuff samples from the upper part of the Hinoshima Formation do not show a distinct youngest peak of eruption age but characteristics of detrital zircons suggestive of maximum depositional age of the host sediments. These results demonstrate that the mean age of the youngest zircon age cluster of a tuff sample does not always indicate depositional age of the tuff, and statistical evaluation of age data is effective to determine depositional age of a tuff bed using zircon U–Pb ages.     

New K–Ar ages of shield lavas from Waianae Volcano, Oahu, Hawaiian Archipelago

A geochronological study utilized the unspiked potassium–argon (K–Ar) technique to obtain ages from the two main volcanic members of the shield stage of the Waianae Volcano, HI. These new dates are further constrained using a combination of stratigraphic relationships, magnetostratigraphy and major element geochemistry. Exposed shield lavas encompass 0.85 Ma, with reliably dated tholeiitic lavas from the main shield ranging from 3.93±0.08 to 3.54±0.04 Ma, and a later shield stage ranging in age from 3.57±0.04 to 3.08±0.04 Ma. These data suggest that the total extent of Waianae shield activity was significantly more than 1 Ma. The age of faulting in two flank zones is constrained to be about 3.4 Ma. Preliminary estimates of lava accumulation rates vary from about 0.3 to 2.0 mm/a; calculated rates show no systematic variation with location in the volcano or with time.     

Application of the Takagi–Sugeno fuzzy system for combination forecasting of river flow in semiarid mountain regions

Because of high spatial heterogeneity and the degree of uncertainty about hydrological processes in large‐scale catchments of semiarid mountain areas, satisfactory forecasting of daily discharge is seldom available using a single model in many practical cases. In this paper the Takagi–Sugeno fuzzy system (TS) and the simple average method (SAM) are applied to combine forecasts of three individual models, namely, the simple linear model (SLM), the seasonally based linear perturbation model (LPM) and the nearest neighbour linear perturbation model (NNLPM) for modelling daily discharge, and the performance of modelling results is compared in five catchments of semiarid areas. It is found that the TS combination model gives good predictions. The results confirm that better prediction accuracy can be obtained by combining the forecasts of different models with the Takagi–Sugeno fuzzy system in semi‐arid mountain areas. Copyright © 2009 John Wiley & Sons, Ltd.